1. Field of the Invention
The present invention relates to a fluid suspension system for a vehicle.
2. Description of the Prior Art
Vehicle suspensions isolate vehicles and their loads from jarring movements or shocks resulting from driving over rough roads and terrain. Isolating shocks reduces both wear and tear on the vehicle body as well as stress on the driver and cargo from shocks transmitted from the wheels to the vehicle body. Suspensions cushion the vehicle by absorbing energy impulses from shocks and vibrations, converting the movements into slower, gentler movements of the vehicle body and dissipating the energy impulses in the moving vehicle body in an unobtrusive way.
The shock or energy converting elements of suspension systems are springs. Springs are commonly associated with and adjacent to each wheel of the vehicle to cushion the vehicle body. An upward shock applied to a wheel is temporarily absorbed by the compression of the adjacent spring. The shock is then transmitted by the spring to the vehicle body as a upward force, resulting in a relatively gentle upward movement of the vehicle. The vehicle body then settles back on the spring which compresses and returns energy to the spring. Shock absorbers dissipate this energy by converting the energy to heat by friction.
A spring must have a sufficient weight bearing capacity to support the vehicle. A spring must carry a vehicle""s maximum load while preventing the transmission of vibration and shock from the wheels to the frame, especially when the vehicle is empty. The spring rate is the relationship between load and spring deflection defined as the load in pounds divided by the deflection of the spring in inches. A soft spring has a low spring rate and deflects a greater distance under a given load. Therefore the selection of a particular spring frequently forces a compromise between choosing a high spring rate for superior handling and low spring deflection and a low spring rate to assure a smooth, soft ride.
Metal springs generate force by moving or deflecting from an at rest position. For compressive springs, such deflection occurs by compressing the spring. Metal springs have a specific deflection. A metal spring has a limit to its deflection and as a result, a limit to its weight bearing capacity. By increasing the spring rate or stiffness of the metal spring, the carrying capacity of the spring increases for a fixed travel or deflection of the spring. However, increasing the spring rate reduces the spring""s ability to absorb energy from road shocks giving a firmer ride. Road noise and abrupt shocks will increasingly be transmitted to the vehicle body and subsequently, the driver and cargo.
Not all springs are metal. Air springs have a number of advantages over metal springs and are increasingly popular. Rather than having a specific deflection like metal springs, air springs hold a load at a given height through the force developed from increased pressure within the spring. For a motor vehicle, the compromise is less drastic between choosing a high spring rate for superior handling and low spring deflection travel and a low spring rate to assure a smooth, soft ride. The advantages of air springs for trucks and buses is greater than for automobiles, due to the widely varying loads carried by trucks and buses.
Rather than merely being a reflection of load to deflection, spring rates in air springs relate to pressure in the air spring and air spring volume. The force required to deflect the air spring increases with greater deflection, because the air is compressed into a smaller space which builds up pressure and resists further deflection. The spring rate can be adjusted to suit conditions, such as load, road conditions or the driver""s personal preference and maintained during various weather conditions. Air spring suspensions can be built which adjust air pressure in the springs to assure that the vehicle always rides at the same height. Thus, the spring rate of an air spring can be adjusted to support any load at a fixed deflection.
Such flexibility does not come without a price, however. If spring rates are adjusted as a function of load, especially to maintain a constant vehicle height, the vehicle""s ride becomes firmer as load increases. For example, it is frequently desirable to have the floor of a vehicle, such as a trailer, a bus or a van, as low as possible. The lower operating height of a low cargo floor, for instance, efficiently transports cargo by providing more useable, internal space for given exterior dimensions. Also, a low floor placed close to the road surface, such as a bus, allows easier access to the vehicle. On buses, especially airport buses where a low ride height for quick and easy loading of passengers is desirable, the frequent presence of standing passengers dictates a smooth ride for passenger safety. To compensate for the increasing passenger load, however, the spring rate increases causing a firmer, less smooth ride. To maintain a smooth ride for passengers despite increasing loads, the volume of the air spring is increased.
One way to avoid a dependency of spring rate to load in air springs has been to introduce an external fluid reservoir in fluid communication with the air spring system. Communication between the air spring and the reservoir may be opened full time or adjusted as required by conditions. When a change in spring rate is desired, the volume reservoir connected to the air spring chamber temporarily alters the apparent volume of the air spring and thus changes the spring rate. This alteration reduces the spring rate while sufficiently inflating the spring to support the load at the desired height.
The prior art external volume reservoirs have two major disadvantages. First, they are too bulky and space intensive to be located close to the air spring, especially when the floor height is low and such reservoirs are more valuable. Second, locating the volume reservoir at some distance decreases or nearly eliminates the reservoir""s advantages by introducing air flow restrictions and friction in the conduit connecting the distant reservoir to the air spring. These restrictions decrease the available volume and the actual spring rate changes from using the air spring with an external reservoir.
Besides prohibiting the use of bulky reservoirs near the air spring, lower operating heights also constrain air spring suspensions by limiting the height at which the air spring operates. Low operating heights limit the maximum volume of air available to compress in lobe air springs, thus preventing the use of taller or xe2x80x9csofterxe2x80x9d air springs. The lower operating heights also limit the compressed height of the air spring. A suspension with a low operating height and a high bump or jounce travel requirement is limited to the use of a short air spring with a short piston. This reduces the benefit of joining the piston air volume with the rubber section air volume.
The prior art has attempted to address the reservoir problem by using an air spring with a hollow piston. The hollow piston serves as a reservoir to increase the apparent volume of the air spring. This air spring uses the air volume in the piston by exchanging air between the air volume in the rubber section of the air spring and the air volume in the piston to provide an air spring with reduced stiffness. Unfortunately, by reducing the size of the air spring for low operating heights, this air spring does not provide a sufficient reservoir of air between the piston and the rubber section to sufficiently cushion a ride in a variety of vehicle and road conditions.
Similarly, U.S. Pat. No. 4,629,170 attempts to address the reservoir problem by using a dual action air spring with two rubber sections. The ""170 air spring changes the spring rate without changing the load bearing capacity by exerting compression or extension forces in the two rubber sections. Unfortunately, the patent does not address the low operating height constraints. The height of the air spring is still too xe2x80x9ctallxe2x80x9d to fit in a low suspension system and the amount of air is insufficient to cushion a ride for a variety of conditions.
Therefore, one object of the invention is to provide a suspension system for a vehicle. Another object of the invention is to provide a suspension system for vehicles with low operating heights. Still another object is to provide a reservoir of air located near the air spring to avoid the dependency of spring rate to load in an air spring system. It is another object to provide such a system while maintaining low spring rates over all vehicle loads to assure a soft or smooth riding vehicle.
According to the invention there is provided a compressible fluid suspension system for a vehicle. The compressible fluid suspension system includes an axle with a reservoir in fluid communication with a compressible fluid spring, such as an air spring. The reservoir provides the spring with an auxiliary volume of compressible fluid. At least part of the axle itself serves as the reservoir for the compressible fluid spring. The axle has a spring interface connecting the axle with the compressible fluid spring. This spring interface is in fluid communication with the reservoir and the spring.
Additional effects, features and advantages will be apparent in the written description that follows.